NANO EXPRESS Open Access Microstructure and adhesion characteristics of a silver nanopaste screen-printed on Si substrate Kwang-Seok Kim 1 , Yongil Kim 2 and Seung-Boo Jung 2* Abstract The microstructural evolution and the adhesion of an Ag nanopaste screen-printed on a silicon substrate were investigated as a function of sintering temperature. Through the two thermal analysis methods, such as differential scanning calorimeter and thermo-gravimetric analysis, the sintering conditions were defined where the temperature was raised from 150°C to 300°C, all with a fixed sintering time of 30 min. The microstructure and the volume of the printed Ag nanopaste were observed using a field emission scanning electron microscope and a 3- D surface profiler, respectively. The apparent density of the printed Ag nanopaste was calculated depending on the sintering conditions, and the adhesion was evaluated by a scratch test. As the sintering temperature increased from 150°C to 300°C, the apparent density and the adhesion increased by 22.7% and 43%, respectively. It is confirmed that the printed Ag nanopaste sintered at higher temperatures sho wed higher apparent density in the microstructural evolution and void aggregation, resulting in the lower electrical resistivity and various scratched fractures. Keywords: silver nanopaste, screen printing, sintering, density, adhesion. Introduction Micro and nanofabrication are essential for the modern electronic devices [1]. Recently, printed electronics has been highlighted by many researchers in academia and industry as emerging manufacturing technologies to fabri- cate portable and display devices [2-6]. The fabrication methods of printed electronics reported so far include direct printing techniques such as inkjet, gravure, and screen printing [7-9]. These techniques have been put for- ward as alternative methods for patterning conducting cir- cuits due to the short manufacturing time, low cost, large- area patternability, and environmental friendliness com- pared to conventional photolithography [10]. Printed elec- tronics is based on an additive manufacturing technology and thereby requires heat treatment after the printing pro- cess. In addition, the features of the patterns directly printed on a substrate also depend on the heat treatment. Therefore, it is essential to understand the behaviors of nanoparticles in a sintering process in order to provide an insight into the printing techniques. Part of an ongoing research project in our laboratory is to produce printed thin films with sufficient adhesion, which is di rectly related to the lifetime of the electro nic devices. However, it is difficult to measure the adhesion of a printed film that has a weak and thin layer, and hence, this has been one of the key issues in this project. A scratch test is the most practical method for assessing the adhesion of the thin film to the substrate [11]. This is because the critical load determ ined by the scratch test is widely regarded as the r epresentative of film adhesion [12]. Based on these requirements, we investigated the effects of heat treatment on the microstructural evolu- tion and electrical property of the screen-printed Ag nanopaste. The influence of sintering temperature on the adhesion was also characterized by the scratch test. Methods The Ag nanopaste (Silver nanopaste DGP, Advanced Nano Materials Inc., Kumho-ri Cheongwon-gun, South Korea) was composed of Ag nanoparticles with a mean size of 24 nm, which were dispersed in a-terpineol matrix at a solid loading of 73% by weight. The shape of the Ag nanoparticles was examined by a JEOL JEM-1200EX * Correspondence: sbjung@skku.ac.kr 2 School of Advanced Materials Science and Engineering, Sungkyunkwan University, 300 Cheoncheon-dong, Jangan-gu, Suwon, 440-746, Republic of Korea Full list of author information is available at the end of the article Kim et al. Nanoscale Research Letters 2012, 7:49 http://www.nanoscalereslett.com/content/7/1/49 © 2012 Kim et al; licensee Springer. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creative commons.org/licenses/by/2.0), which permits unrestrict ed use, distribution, and reproduction in any medium, provided the original work is properly cited. transmission electron microscope [TEM] (JEOL Ltd., Akishima, Tokyo, Japan). Two types of thermal analysis were performed on the Ag nanopaste: differential scanning calorimeter [DSC] and thermo-gravimetric analysis [TGA]. A screen printing machine (MT-550TV, Micro- Tec, Chiba City, Chiba, Japan) was used to duplicate the conductive patterns. The Ag nanopaste was printed using a 400-mesh screen mask onto a silicon [Si] substrate passi- vated with SiO 2 . All printed patterns were dried on a hot plate at 70° C for 10 min, and then sintered in a box-type, muffle furnace (RTA-BRT100, BLS Korea Inc., Seoul, South Korea) for 30 min under a sintering temperature ranging from 150°C to 300°C. A four-point probe method was adopted to measure the electrical resistivity. The apparent density of the screen-printed Ag nanopaste depending on the sintering temperatures was calculated from the precise volume and mass measured using a com- mercial precision scale (JL-180, Chyo Balance Corp., Min- ami-ku, Kyoto, Japan) and a 3-D surface profiler (Nanoview2000, Nanosystem Inc., Daejon City, South Korea), respectively. Figure 1 shows a schematic diagram of scratch testing, and the table in Additional file 1 lists the detailed scratch parameters. The scratch test was car- ried out using a commercial scratch tester (MSTX, CSM Instruments, Needham, MA, USA) equipped with a dia- mond indenter having a tip radius of 10 μm. The micro- structural evolution was investigated using a field emission scanning electron micros cope [FE-SEM] (JSM-7401F, JEOL Ltd., Akishima, Tokyo, Japan), and an optical micro- scope [OM] was used to observe the fracture surfaces of the screen-printed Ag nanopaste after the scratch test. Results and discussion Figure 2a, b shows a TEM image and the measured size distribution of the Ag nanopaste, respectively. In the TEM image, most of the Ag nanoparticles have a dia- meter of approximately 25 nm. It was also confirmed that the diameters of the A g nanoparticles were within 10 and 30 nm with a narrow distribution, as shown in Figure 2b. This distribution of particle size was known to be affected by three major factors, i.e., the generation rate of the Ag embryos, the growth rate of the Ag nano- particles, and the extent of surfactant absorbing or encapsulati ng if no agglomeration occurred between the nanoparticles [13]. The thermal properties of the Ag nanopast e are shown in Figure 3. The DSC curve had a slightly exothermic slope from 30°C to 200°C at the rate of 10°C/min and one sharp endothermic peak a t 230°C. It means that the necking reaction in the Ag nanopaste occurred at the point of the endothermic peak, resulting in the coarsened nanoparticles. Compared with the TGA curve, any moist- ureandsolventintheAgnanopasteweredriedor decomposed thermally from 100°C to 300°C at the rate of 10°C/min. In this temperature range, a maximum rate of weight loss was shown at 225°C, and the total weight loss was approximately 27 wt.%. Based on these results, the sintering temperature range was determined from 150°C to 300° C to investigate the influence of heat treatment on the microstructural evolution and property variation. The microstructural evolution as a function of sinter- ing temper atures was shown in Figure 4. The surf ace of the Ag nanopaste sintered at 150°C for 30 min exhibited Figure 1 A Schematic diagram of scratch testing. Figure 2 A TEM image (a) and s ize distribution (b) of Ag nanoparticles. Kim et al. Nanoscale Research Letters 2012, 7:49 http://www.nanoscalereslett.com/content/7/1/49 Page 2 of 6 a similar particle shape and size compared to the as- dried one. However, when the printed Ag nanopaste was sintered at 200°C for 30 min, the clusters were built via interconnections, resulting from interparticle necking that occurred after the drying of the dispersing agent and decomposition of the organic solvent. Therefore, this microstructural evolution matched well with the thermal analysis results, as shown in Figure 3. Above a sintering temperature of 200°C, the s urface of the sin- tered Ag nanopaste drastically changed from discrete and spherical Ag nanoparticles to continuous and con- solidated ones. Figure 5 plots the variation in the apparent density of the Ag nanopaste as a function of sintering temperature. Except at 300°C, the average apparent density did not change much which was approximately 6.32 g/cm 3 within the standard deviation of 0.22. At 300°C, the film of the Ag nanopaste as the result of volume shrinkage appear ed after the interparticle neck growth, as shown in Figure 4d [14]. Because the apparent density w as calcu- lated from the values of the measured mass and volume, any change in mass or volume would consequently affect the density. Although some pores were observed inside the film of the Ag n anopaste, the main parameter was the volume shrinkage of the screen-print ed Ag film com- pared to the mass decrement caused by the solvent eva- poration. In this case, although the mass of the Ag nanopaste was reduced by solvent evaporation that resulted in observable pores inside the film, the dominat- ing factor was still the volume shrinkage of the screen- printed Ag film. Therefore, the volume shrinkage led to the increase of apparent density. Figure 3 DSC (a) and TGA (b) curves of the Ag nanopaste. Figure 4 FE-SEM micrographs of the screen-printed Ag nanopaste sintered at various temperatures.(a, e) 150°C, (b, f) 200°C, (c, g) 250°C, and (d, h) 300°C. Kim et al. Nanoscale Research Letters 2012, 7:49 http://www.nanoscalereslett.com/content/7/1/49 Page 3 of 6 Figure 6 reveals the influence of the sintering tem- perature on the electrical resistivity of the Ag nanopaste. The electri cal resistivity dramatically decreased with the incr easing sintering temperature, implying a cond uction pathway between the Ag nanoparticles due to the inter- particle neck formation and growth. This is because the mechanism of electrical conduction in a metal nano- paste features the point-to-point contact between the conductive nanoparticles [15]. In other words, when the conductive nanoparticles have been necked to a suffi- cient extent, the film of the printed nanopaste becomes relatively well conduct ive despite of its poros ity. The Ag nanopaste sintered at 300°C showed the lowest electrical resistivity of 1.89 μΩ·cm. Figure 7 exhibits the adhesion strength of the Ag nanopaste printed on the Si substrate as a function of sintering temperature. A critical friction force is the force needed to pull out a film on a substrate. Overall, the critical friction force increased linearly with an increasing sintering temperature. As previously seen in Figure 4b, t he nanoparticles on t he surface of the Ag nanopaste formed clusters with a diameter of 130 to approximately 180 nm due to interparticle necking at a sintering te mperature of 200°C. The clusters grew larger with the three-dimensional interconnections as the sin- tering temperature increased, which increased from around 300 nm at 250°C (Figure 4c) to 600 nm at 300°C (Figure 4d). Therefore, as the sintering temperature increased, the clusters were connected more strongly, and hence, the surf ace area be tween the Ag nanopaste and the Si substrate increased, resulting in the higher friction force. In order to investigate the scratched surface of the Ag nanopaste, the scratches were observed using the OM. The panorama images illustrate the direction of scratch testing, and the morphologies of the entire scratches are shown in Figure 8a, b, c, d. As the sintering temperature increased, larger parts of the Ag nanopaste were pulled out due to the stronger connections in the Ag clusters. Figure 8e, f, g, h indicates the exact starting points to pull the film out from the substrate. The different fracture modes were identified depending on the sintering tem- perature. The printed Ag nanopaste sintered at 300°C exhibited a fracture behavior like a bulk Ag film. Conclusions The characteristics of thin printed patterns are domi- nated by the heat treatment applied. The influence of sin- tering temperature on the adhesion of the screen-printed Ag nanopaste was investigated. The scratch test, which is to measure the critical friction force of the film, was sug- gested to be a suitable method to evaluate the adhesion of printed patterns. Overall, the critical friction force increased by 43% as the sintering temperature increased Figure 6 The electrical resistivity of the Ag nanopaste. Figure 7 The critical friction force of the Ag nanopaste. Figure 5 The apparent density of the Ag nanopaste. Kim et al. Nanoscale Research Letters 2012, 7:49 http://www.nanoscalereslett.com/content/7/1/49 Page 4 of 6 from 150°C to 300°C. To rationalize these experimental results, the microstructural evolution and variation of density were investigated as a function of sintering tem- perature. The Ag nanopaste sintered at higher tempera- tures showed the accelerated condition. The calculated apparent density of the Ag nanopaste increased from 6.08 g/cm 3 at 150°C to 7.46 g/cm 3 at 300°C. It was con- cluded that the printed Ag films sintered at higher tem- peratures became more densely packed, which resulted in the lower electrical resistivity and the stronger adhe- sion of the printed Ag nanopaste. Additional material Additional file 1: The parameters of scratch test. A table listing the detailed scratch parameters. Abbreviations DSC: differential scanning calorimeter ; FE-SEM: field emission scanning electron microscope; OM: optical microscope; TEM: transmission electron microscope; TGA: thermo-gravimetric analysis Acknowledgements This work was supported by the World Class University program through the National Research Foundation of Korea funded by the Ministry of Education, Science, and Technology (Grant No. R32-2009-000-10124-0). Author details 1 SKKU Advanced Institute of Nanotechnology (SAINT), Sungkyunkwan University, 2066 Seobu-ro, Jangan-gu, Suwon-si, Gyeonggi-do, 440-746, Republic of Korea 2 School of Advanced Materials Science and Engineering, Sungkyunkwan University, 300 Cheoncheon-dong, Jangan-gu, Suwon, 440- 746, Republic of Korea Authors’ contributions KSK carried out the density measurement and scratch test and wrote the manuscript. YK carried out the two thermal analysis of the Ag nanopaste and participated in the screen printing. SBJ participated in the design and coordination of this research. All authors read and approved the final manuscript. Competing interests The authors declare that they have no competing interests. Received: 1 September 2011 Accepted: 5 January 2012 Published: 5 January 2012 References 1. Yoo H, Shin H, Lee M: Direct patterning of double-layered metal thin films by a pulsed Nd:YAG laser beam. Thin Solid Films 2010, 518:2775-2778. 2. Kim KS, Lee YC, Ahn JH, Jung SB: The effect of heat treatment on flexibility of silver nanopaste circuit screen-printed on polyimide. IEEE NMDC2010: Topics in Nanostructure Characterization. IEEE Nanotechnology Materials and Devices Conference (NMDC)2010; October 2010 Monterey: IEEE; 2010, 306. 3. Ko SH, Pan H, Grigoropoulos CP, Luscombe CK, Frechet JMJ, Poulikakos D: All-inkjet-printed flexible electronics fabrication on a polymer substrate by low-temperature high-resolution selective laser sintering of metal nanoparticles. 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Submit your manuscript to a journal and benefi t from: 7 Convenient online submission 7 Rigorous peer review 7 Immediate publication on acceptance 7 Open access: articles freely available online 7 High visibility within the fi eld 7 Retaining the copyright to your article Submit your next manuscript at 7 springeropen.com Kim et al. Nanoscale Research Letters 2012, 7:49 http://www.nanoscalereslett.com/content/7/1/49 Page 6 of 6 . microstructural evolution and the adhesion of an Ag nanopaste screen-printed on a silicon substrate were investigated as a function of sintering temperature. Through the two thermal analysis methods,. NANO EXPRESS Open Access Microstructure and adhesion characteristics of a silver nanopaste screen-printed on Si substrate Kwang-Seok Kim 1 , Yongil Kim 2 and Seung-Boo Jung 2* Abstract The. electrical property of the screen-printed Ag nanopaste. The influence of sintering temperature on the adhesion was also characterized by the scratch test. Methods The Ag nanopaste (Silver nanopaste